I mislead you ..... I'm already using buffalo as wired Access Point, I configured it at the time I bought the TIM banded ZTE MF286D.
I have the openwrt router as 192.168.100.1 while the dd-wrt access point as 192.168.100.201. I set up this address (201) on dd-wrt and I think I should see it on openwrt router as connected device (as other connected devices).
Thanks